wave packet
A wave packet is a localized bundle of waves — a wavefunction that is concentrated in a small region of space rather than spread out everywhere. It is built by adding together many simple waves of slightly different wavelengths, which reinforce one another in one compact region and cancel out elsewhere. The result is the quantum way to represent a particle that has a fairly well-defined location and a fairly well-defined momentum at the same time.
Because a wave packet is a sum over many wavelengths, it embodies the uncertainty trade-off directly. A tightly localized packet needs a wide spread of wavelengths, hence a broad range of momenta; a packet with a narrow spread of momenta must be long and spread out in space. The width in position times the width in momentum can never beat the ℏ/2 floor, with a smooth Gaussian packet achieving the best possible compromise.
Wave packets are how physicists reconcile the wave picture of quantum mechanics with the everyday image of a particle moving along. The central peak of the packet travels at the group velocity, which matches the classical velocity of the particle, so a localized packet glides along much as a little ball would. It is the closest the wave description comes to a familiar moving object, while still being honest that the object is really a spread-out probability amplitude.
Adding waves of many wavelengths makes a localized bundle that moves at the group velocity.
The wave packet is a probability amplitude, not a little lump of physical stuff travelling through space. Its squared magnitude gives the probability density of where the particle would be found if measured, following the Born rule.